The present invention relates generally to lasers based on periodic structures having photonic band gaps, and more particularly to lasers having selectable lasing modes.
Semiconductor lasers have found many industrial and commercial applications in recent years. For example, lasers are used in telecommunications, in optically readable media pickups that are used in CD players, CD ROM drives and DVD players, and in medical imaging. However, previously known semiconductor lasers have a number of disadvantages. For example, traditional semiconductor lasers, such as ones used in CD players, emit light from the edge of a chip, so it is necessary to cleave a wafer into chips and package the chip before knowing if the laser functions properly. Other types of light sources, such as LEDs do not provide the performance needed for certain applications.
Vertical Cavity Surface Emitted Lasers (hereinafter xe2x80x9cVCSELsxe2x80x9d) have been developed to address the need for a more advanced, higher quality laser that can function well in a variety of applications. VCSELs combine the performance advantages of LEDs and of edge-emitting lasers at costs comparable to LED solutions. VCSELs emit light vertically from the wafer surface, like LEDs, which means their fabrication and testing is fully compatible with standard I.C.s procedures and equipment, and also means that arrays of VCSELs are feasible. Additionally, VCSELs are much faster, more efficient, and produce a smaller divergence beam than LEDs.
The VCSEL structure leads to a host of performance advantages over conventional semiconductor lasers.
1) small size
2) low power consumption
3) 2-dimensional array capabilities
In contrast to conventional edge-emitting semiconductor lasers, the surface-emitting VCSEL has a symmetric Gaussian near-field, greatly simplifying coupling to optical elements or fibers. In addition, VCSEL technology allows the fabrication of two-dimensional laser arrays.
However, VCSELs suffer from a number of disadvantages. The manufacture of VCSELs requires sophisticated and expensive microfabrication. Since single-pass gain in thin layer semiconductor lasers is low, VCSELs incorporate highly reflective dielectric stacks which are integrated into the laser as Bragg reflectors. These consist of alternating layers of dielectric material, which are grown using methods of molecular beam epitaxy (MBE). This ensures a close match of the atomic lattice structures of adjacent layers. Alternating atomically ordered layers of materials with different electronic characteristics are thereby produced. The interfaces between the layers must be digitally graded and doped to reduce the electrical resistance.
Much work has been done to improve the performance of VCSELs by increasing the number of layers and/or the dielectric difference between alternating layers. However, this approach makes the fabrication more expensive and difficult. There is also a limit to the number of layers determined by the absorption in these layers. While VCSELs can be manufactured in two-dimensional arrays, there has been great difficulty in achieving uniform structure over large areas and in producing large area arrays. The materials typically used for VCSELs do not have the desired low absorption and high index contrast over a broad frequency range. In particular, it is difficult to achieve high reflectivity in the communication band around 1.5 microns.
In addition, VCSELs cannot be tuned in frequency since their periods cannot be changed. The density of photon modes is not changed appreciably by use of low index contrast multilayer Bragg reflector and the gain cannot be improved in a VCSEL system as compared to that in an ordinary laser cavity. Also, an external device must be used to control the polarization of the light.
In recent years, periodic materials, for example chiral materials such as cholesteric liquid crystals (CLCs), have been used in a variety of lasing, filtering and other similar applications to address common drawbacks of standard semiconductor devices such as VCSELs. Photonic band edge lasing has recently been demonstrated in dye-doped cholesteric materials with emission being in a narrow range of frequencies near the edge of the photonic band gap and with the same periodicity as the CLC structure. At the lowest excitation power provided by an external excitation source (such as an optical pump), lasing was produced at the photonic mode closest to the edge of the photonic band gap for radiation propagating perpendicular to the molecular planes of the chiral material with the same sign of rotation as a molecular director of the CLC structure. As the pump power was increased, however, to the point where the efficiency of the laser was high, the laser bandwidth broadened while emission spectra appeared to be a set of individual peaks resulting in loss of efficiency. Other periodic lasers experience similar difficulties. Furthermore, selection of lasing wavelength in previously known periodic lasers is often a difficult task.
It would thus be desirable to provide an apparatus and method for advantageously enabling single-mode lasing at higher pump power, for generally reducing the bandwidth of the lasing radiation, and for enabling advantageous selection of a particular photonic mode for lasing at that mode in a periodic laser. It would also be desirable to provide a laser apparatus and method that has advantageous properties similar but superior to VCSELs and that has none of the VCSELs"" disadvantages
The present invention advantageously enables single-mode lasing at higher pump power and further enables general reduction of the lasing radiation bandwidth. This is accomplished by producing an appropriate spatial gain distribution inside the laser medium. The present invention further enables advantageous selection of a particular photonic mode for lasing at that mode in a periodic laser by varying spatial gain distribution within the periodic laser medium.
Photonic mode selection by varying gain distribution within the laser medium is accomplished in accordance with various embodiments of the present invention. In a first embodiment of the present invention, a periodic structure having a photonic band edge and being doped with a uniformly distributed gain medium, is optically pumped at a wavelength and polarization of a selected high photonic mode at or near the high-frequency band edge to produce lasing at a frequency of a corresponding low photonic mode of the same order at or near the low-frequency band edge.
In a second embodiment of the present invention, a periodic structure having a photonic band edge and being doped with a uniformly distributed gain medium, is optically pumped by a gaussian light beam that is focused by a lens in the middle of the periodic structure. This arrangement produces a maximum excitation density in the middle of the periodic structure and favors lasing in the first photonic mode from either the high or low frequency band edge. The particular edge favored in lasing is selected by varying the relative strength of the pumping beam and the gain spectral distribution of the gain medium.
In a third embodiment of the present invention, a periodic structure is doped with gain medium disposed within the structure in a non-uniform manner, thereby producing a spatially modulated gain medium. The number of high density gain medium areas in the periodic structure as well as the gain spectral distribution determine the number, and thus the wavelength, of the photonic mode at which lasing occurs.
In a fourth embodiment of the present invention, a periodic structure having a photonic band edge and being doped with a uniformly distributed gain medium, is electrically excited by a spatially non-uniform electrical current resulting in a modulated gain within the periodic structure. The peak gain distribution as well as the gain spectral distribution within the structure determine the number, and thus the wavelength, of the photonic mode at which lasing occurs.
In a fifth embodiment of the present invention, a periodic structure having a photonic band edge and being doped with a uniformly distributed gain medium is arranged as a vertical cavity liquid crystal laser (VCLCL). The VCLCL is optically pumped by a plurality of light emitting diodes (LED) surrounding VCLCL in a spatially non-uniform manner to produce one or more high gain peak areas within the periodic structure. The number of high gain peaks in the periodic structure as well as the gain spectral distribution determine the number, and thus the wavelength, of the photonic mode at which lasing occurs.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.